Liquid-crystal displays of this type are known in principle, for instance, from U.S. Pat. No. 4,668,049 in the name of Stanley Canter. The activating light here is ultraviolet light, and the cells are scattering cells on a light guide in the form of a TIR (total-internal-reflection) substrate. Each cell when not addressed is essentially transparent and does not affect the passage of the ultraviolet light, which therefore remains contained within the light guide so that the corresponding phosphor remains dark. When the cell is addressed it scatters the UV light, some of which therefore escapes the TIR conditions and reaches the phosphors.
Such an arrangement has many advantages, one of which is that narrow-band or monochromatic light can be used as the activating light; this avoids many of the limitations engendered by the wavelength-dependent optical properties of liquid crystals, while making colour displays perfectly possible if the appropriate phosphors are used. Also the viewing-angle problem typical of LC displays is eliminated because the secondary light is emitted by the phosphors and does not have to pass through the liquid-crystal layer.
A problem that nevertheless remains with this kind of display, henceforth photoluminescent LC display or PLLCD, is that to produce an accurate image in the phosphor plane of the image written into the LC cells in the shutter plane, the beam of UV (activating light) emerging from each shutter should be incident solely on the appropriate phosphor. Moreover, in order to maximise the efficiency of this stage of the UVLCD it is also important that the maximum of the beam cross-section is incident on the phosphor (as opposed to on a mask screen between the phosphors, for instance).
EP-A-185495, corresponding to U.S. Pat. No. 4,668,049 mentioned above, purports to address the problem of directing the activating light at the phosphors, namely on page 18 referring to FIG. 5 of that application. However, although various strategies are mentioned, such as reducing the thickness of the front glass and including blocking layers between the pixels, it is clear that a solution has not been found: even with the thinnest glass practicable, say 100-200.mu., its thickness would be at least comparable with the spacing of the phosphors, about 200.mu. for a high-resolution monitor; since for a scattering device the light supplied must be incident at a shallow angle for total internal reflection to take place in the off-state, it is impossible to prevent scattered light striking adjacent pixels.
An approach to solving this general problem is disclosed in WO 95/27920 (Crossland et al.). This shows the use of means for collimating the activating light between the light source and the liquid crystal, or between the liquid crystal and the phosphors. This approach can solve the problem but involves additional components.